The present invention relates to the field of ultra-high frequency ion sources, usable both in electron sources and in plasma generators.
It has numerous applications in the field of the sputtering of thin films, microetching, ion implantation, electron irradiators, heating by a beam of fast neutral particles of the plasma of thermonuclear reactors, tandem accelerators, synchrocyclotrons as well as in surface cleaning and treatment.
Hitherto ultra-high frequency ion sources have been based on the principle of electron cyclotron resonance ion sources. In such known sources, the ionization of a neutral gas and the appearance of a plasma are brought about by combining in an hf cavity, the synergetic effects of an ultra-high frequency electromagnetic field of frequency F and a constant magnetic field B, so as to obtain resonance in said latter frequency and the pulsation .omega.=eB/m of the electrons in their circular paths around the force lines of field B. Thus, this condition is written: F=eB/2.pi.m, in which e and m are the electron charge and mass, B the constant magnetic field present in the cavity and which gives the relation linking f and B to obtain electron cyclon resonance within the plasma. The electrons then describe spiral paths around the force lines of field B by absorbing the energy of said field and by thus acquiring a maximum kinetic energy for bringing about ionization by impacts of the neutral gas molecules present in the source hf cavity.
Such ion sources are described by R. Geller, C. Jaquot and P. Sermet in "Proceedings of the symposium on ion sources and formation of ion beams", Berkeley, October 1974 and F. Bourg, R. Geller, B. Jacquot, T. Lamy, M. Pontonnier and J. C. Rocco in "Nuclear instruments and methods", North-Holland Publishing Company, 196, 1982, pp. 325-329. They are based on establizhing a confinement of the plasma with the aid of a mirror magnetic configuration and maximum values of the magnetic field B higher than the value ensuring electron cyclotron resonance.
The correct operation of such known ion sources still requires certain special precautions, and involves a high electrical energy consumption, particularly for producing constant magnetic fields, for bringing about the electron cyclotron resonance and for the extraction of the ions. Thus, for example, in a known ion source of this type, the maximum and minimum magnetic induction values are 0.42 and 0.32 Tesla respectively, and electron cyclotron resonance takes place at 0.36 Tesla, the frequency of the high frequency wave injected being fixed at approximately 10 GHz.
Thus the ions produced in the plasma are extracted by an extraction system constituted by electrodes raised to d.c. potentials and which are downstream of the maximum of the magnetic field. Under these conditions, ion current emitted by the source decreases in proportion to the value of the field of the extraction point and, to obtain an intense ion current, it is necessary to extract the ions in a magnetic field at least of the same order of magnitude as the cyclotron resonance field.
This necessity is unfortunately incompatible with a good optical quality of the extracted ion beam on cancalling out the magnetic field prior to the impact of the ions in the utilization zone. Thus, in this case the ions take on transverse energy, the beam diverges and its optical qualities are reduced, in accordance with the effect described in the Bush theorem.
In order to retain the optical qualities of the beam downstream of the ion source, it is then necessary to keep the magnetic field constant throughout the drift space of the ion beam, up to the point of its application for the transformation of the ions into neutral particles. For the example described hereinbefore, the field to be kept constant corresponds to an inducation of approximately 0.36 Tesla and the electrical power consumed by the coils producing this magnetic field is approximately 1 megawatt.
In the case of using low energy ions, (below 1 kev) the extraction system does not make it possible to extract the high densities. In order to increase the latter, it is possible to compress the ion beam downstream of the ion source. In order to compress the ion beam, the magnetic field must be incresed in proportion thereto. The increase in the current density of the ions obtained is consequently limited by the technical problems occurring with respect to the production of magnetic fields with this order of magnitude.
Thus, in summarizing, the ion sources according to the prior art have the main disadvantages of a very high energy consumption for establishing the magnetic configuration, whilst the increase in the density of the low kinetic energy ion current is problematical, due to the need of a high magnetic field for transferring the latter downstream of the extraction to the place of use.
In order to obviate these disadvantages, a number of solutions have been proposed, such as e.g. that described in French patent application No. 83 08401 of the Commissariat a l'Energie Atomique of May 20th 1983.
In this unpublished French application, the magnetic confinement configuration has been modified in order to permit the extraction of ions in a magnetic field much smaller than that of the prior art sources. This has led to a considerable economy with respect to the energy required for producing the magnetic configuration in the source and also downstream of the extraction, during low energy transfer of the extracted beam.
However, the ion source described in this patent application still uses electron cyclotron resonance for producing the plasma in the cavity, so that it still requires therein the presence of a magnetic field, which is higher or at least equal to that producing the electron cyclotron resonance.